The present invention relates to a discharge lamp driving apparatus which can dim a discharge lamp and a lighting fixture.
A compact fluorescent lamp used for general lighting has a folded portion that is formed into a U-shape or H-shape at the end of the lamp. A fluorescent lamp is useful as a light source that is compact in size and capable of obtaining a high luminance. FIG. 10 shows diagram of a driving apparatus for operating a folded discharge lamp with a high frequency power. Reference numeral 31 denotes, for instance, a 50 Hz, 100V alternating current power source. A high-frequency driving apparatus 32 comprises, for instance, a high-frequency inverter operates at a power supplied from the alternating current power source and lights a folded discharge lamp 33 at a high frequency in tens of kHz.
The high-frequency driving apparatus 32 is capable of making the dimming control continuously in several steps of non-step by varying frequency of a high-frequency power source for operating lamps. The folded discharge lamp 33 comprises electrodes 331, 332 to which a high frequency voltage is applied from the high-frequency driving apparatus 32, two straight tubes 333, 334 arranged in parallel with each other and a communicating tube 337 arranged to a position slightly offset to the electrodes from the ends 335, 336 of the straight tubes 333, 334, for communicating the straight tubes 333, 334.
In the discharge lamp driving apparatus shown in FIG. 10, the lamp current flows through a folded passage formed by two straight tubes 333, 334 and the communicating tube 337 between the electrodes 331, 332 during the normal high-frequency lighting. However, when the folded discharge lamp 33 is operated and dimmed (i.e., decreased brightness) at a high-frequency (reduced the brightness), the lamp current I decreases but the lamp voltage V increases between the electrodes 331, 332, as shown in FIG. 11. When the folded discharge lamp is thus dimmed, high frequency leakage currents (shown by dotted lines in FIG. 10) from one electrode 331 to the other electrode 332 occur across the straight tubes 333, 334 without passing through the straight tubes 333, 334 and the communicating tube 337, due to that positive columns in the two straight tubes 333, 334 are adjacent to each other.
This leakage current becomes larger at a position closer to the electrodes 331, 332, as it is closer to a high frequency power source (i.e., the energy supply source). On the other hand, the leakage current becomes smaller at a position remoter from the electrodes 331, 332. This leakage current effectually acts on the fluorescent material coating of the straight tubes 333, 334. Accordingly, the leakage current is converted to a visible light and contributes to increasing brightness. As a result of such distribution of leakage energy, brightness at the straight tubes 333, 34 of the folded discharge lamp 33 becomes brighter at a position closer to the electrodes 331, 332 and becomes darker when goes away therefrom. That is, the more closer to the tubes 335, 336, lamp current and leakage current flowing through the straight tubes 333, 334 becomes less and the light becomes dark. Moreover, the more the degree of dimming becomes higher, the more the degree of non-uniformity of brightness (non-uniformity of luminance) will become higher. This is because the more the degree of dimming becomes large, the more lamp voltage increases. When the degree of dimming becomes higher and the brightness distribution along the straight tubes 333, 334 becomes non-uniform, the lamp lighting state may become unstable or flickering or the lighting may go out. By the way, there have been adapted other methods that a connector is brought to face the entire surfaces of the discharge lamp tubes or pasted on one side of the discharge tubes in the direction of tube length in order to improve starting characteristic of a discharge lamp or flickering, etc. But no effect could be obtained to prevent non-uniform of luminance and unstable lighting caused from leakage current as in a folded discharge lamp.
As described above, there was such a problem that when a folded discharge lamp is operated at high frequency and dimmed, the light is darkened when going away from the electrodes due to leakage current between adjacent tubes and the light becomes non-uniform of(non-uniform of luminance) along the tubes.
An object of the present invention is to provide a discharge lamp driving apparatus and a lighting fixture that are which can dim stably without causing non-uniform of brightness along lamp tubes when lighting and dimming a folded discharge lamp.
A discharge lamp driving apparatus according to one aspect of the invention comprises, a direct current power supply having a rectifier, an inverter circuit, which has stable and unstable output terminals, converting a direct current power to an alternating current power with a predetermined high frequency and driving a discharge lamp having a folded portion, a controller for being able to dim the discharge lamp, and an auxiliary conductor, arranged to the folded portion of the discharge lamp, and electrically connected to the stable potential output terminal of the inverter.
In another aspect of the invention, a lighting fixture comprises, a lighting fixture body, and a discharge lamp driving apparatus comprising a direct current power supply having a rectifier, an inverter circuit, which has stable and unstable output terminals, converting a direct current power to an alternating current power with a predetermined high frequency, and driving a discharge lamp having a folded portion, a controller for dimming the discharge lamp, an auxiliary conductor, located to the folded portion of the discharge lamp, and electrically connected to the stable potential output terminal of the inverter.